This number here, what does it mean?
That is the cross track error shown on the Navigation Display (ND):
9 Cross Track Error
This is the aircraft's lateral deviation from the active leg of the flight plan (related to the great circle route). It is indicated in nautical miles (NM), with the letter R (right) or L (left), according to the position of the aircraft with respect to the flight plan.
(Airbus A320 FCOM - Indicating/Recording Systems - Indications on ND - Rose NAV mode / ARC mode)
In your example, it indicates that the aircraft is 19.1 NM to the right of the flight plan (shown as the dashed green line). The reason seems to be the presence of moderate and heavy precipitation along the planned route (shown as the yellow and red patches from the weather radar). The flight crew switched to heading or track mode (the solid green line indicates the current track) to manually fly around the weather.